1. Technical Field of the Invention
This invention relates to a crystal-structured semiconductor film, semiconductor device using the same and method for manufacturing those, and more particularly to a semiconductor film excellent in crystallinity and having a crystal orientation arranged in a single direction, semiconductor device using the same and method for manufacturing those.
2. Description of the Related Art
There is a technology, called a laser anneal process, developed as a method to crystallize an amorphous silicon film formed on an insulating substrate of glass or the like. In the laser anneal process, a laser light having an energy of approximately 100–500 mJ/cm2 is radiated to an amorphous silicon film, thereby realizing crystallization.
For amorphous silicon crystallization, there is a need to heat it up to usually 600° C. or higher. The laser anneal process, however, has an extremely excellent feature that can crystallize an amorphous silicon film while keeping a substrate at nearly a room temperature. The laser uses a solid laser, as represented by an excimer laser or a YAG laser. In any way, because of limitation in beam size, the processing of a large-area substrate requires radiation by connection with bean scans. Accordingly, there is a disadvantage pointed out that crystallinity changes at connections thus disabling to obtain a uniform crystal. Meanwhile, in the case of laser anneal, there also is difficulty in obtaining a homogeneous crystal because of instable output of a laser oscillator. Such crystal quality variation is responsible for the characteristic variation in thin film transistors (hereinafter, described as TFTs).
On the other hand, Japanese Patent Laid-Open No. 7-231100, Japanese Patent Laid-Open No. 7-130652, Japanese Patent Laid-Open No. 8-78329, etc. disclose an art that, using a catalyst element for accelerating the crystallization of an amorphous silicon film, a heating process is made at a temperature of 450-650° C. to cause crystallization in a part or the entire of an amorphous silicon film, and heating is further made at a temperature higher than that heating temperature to thereby obtain a large-grained crystalline silicon film.
In order to obtain a high-quality crystalline silicon film, it is emphasized to arrange the orientation of crystals besides the increase of crystal-grain size. It is considered, in the laser anneal process, that crystallization proceeds on the basis of the spontaneous nucleation of crystals at the interface between an amorphous silicon film and a substrate. The silicon film crystallized by this method, when analyzed in the crystal structure by X-ray diffraction, is usually observed with diffraction peaks at (111), (220), (311) and so on. It has been confirmed as a polycrystalline body aggregated with various orientations. In the polycrystalline body, individual crystal grains precipitate on arbitrary crystal planes. In this case, the probability is the greatest that crystal precipitation occurs on a (111) plane where the interface energy is minimized to an underlying silicon oxide.
In the case that a catalyst element for accelerating silicon crystallization is introduced into an amorphous silicon film to cause crystallization, formed is a silicide of an element introduced at a temperature lower than a temperature of spontaneous nucleation, causing crystal growth on the basis of the silicide. For example, NiSi2 under forming does not have a particular orientation. However, in case the amorphous semiconductor film is reduced to a thickness of 200 nm or less, growth is allowed substantially only in a direction parallel with a substrate surface. In this case, minimum is the interface energy at the contact between the NiSi2 and the crystal-silicon (111) plane. Thus, the plane parallel with a crystalline silicon film surface is a (110) plane, in a lattice plane of which preferential orientation made. Where the crystal grows in a columnar form in a direction parallel with the substrate surface, there exists a freedom in a direction about an axis of the columnar crystal. Thus, orientation is not always on the (110) plane and precipitation occurs also on the other lattice planes. The percentage of orientation on the (110) plane is, however, still less than 20 percent in total.
In the case of low orientation ratio, it is almost impossible to maintain a lattice continuity at a crystal boundary where crystals with different orientations crash one against another. Easily presumed is formation of a number of dangling bonds. The dangling bond at a grain boundary acts as a recombination center or trap center, to reduce the property of carrier (electron/hole) transport. As a result, there is a problem that, because the carriers are vanished due to recombination or trapped in defects, high mobility is not to be expected by the use of such a crystalline semiconductor film.
There is a disclosure, in Japanese Patent Laid-Open No. 2000-114172, of an art that crystallization is made by adding a proper amount of germanium to a silicon film in order to enhance crystal orientation ratio. This publication indicates to obtain a semiconductor film that can be substantially considered as a single crystal exhibiting such a crystallinity that individual crystal grains are arranged in plane orientation order despite it is a semiconductor film aggregated with a plurality of crystal grains. In obtaining it, however, a thermal process at 900–1200° C. is required besides the addition of germanium.
In this manner, crystal quality can be improved by carrying out a thermal process at a high temperature exceeding 900° C. However, such a thermal process cannot be carried out for a crystalline silicon film formed on a glass substrate less resistive to heat. Also, there is a problem that, even if the orientation ratio is enhanced by germanium addition, germanium low in combination energy with hydrogen is not easy for hydrogenation. Namely, a hydrogenation process cannot compensate for the dangling bond caused by germanium.
It is an object of the present invention to provide means for solving the foregoing problem, and to enhance the orientation ratio of a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film while using as a substrate a less heat-resistive material such as glass thereby providing a semiconductor device using a crystalline semiconductor film with the high quality equivalent to a single crystal.